1. Field of the Invention
The present invention relates to a gas turbine having a cooling structure of a ring segment disposed on an outside periphery in the radius direction of a movable vane and further having a sealing structure for preventing an invasion of combustion gas into a turbine disc.
2. Description of the Prior Art
As for an entire construction of the gas turbine, as schematically shown in FIG. 15, air is compressed in a compressor 81, fuel is charged to a combustor 82 to generate combustion gas and then the same combustion gas is introduced into a turbine 83 as main stream gas so as to rotate a generator 84.
As generally shown in FIG. 16, the turbine 83 comprises plural rows of static vanes 3 and plural rows of movable vanes 2 which are alternately disposed. An outside peripheral side, in the radius direction (or outside diameter side in the radius direction), of the movable vane 2 is structured so as to be surrounded by plural ring segments or tip seal segments 90 which are divided in the peripheral direction so as to feed a high temperature gas through an appropriate gap between that ring and movable vane 2.
The ring segment 90 has a cooling structure capable of bearing a high temperature main stream gas 15 discharged from the combustor 82. An example of the ring segment 90 having this cooling structure in a conventional gas turbine will be described with reference to FIGS. 8 and 9.
Cooling medium 89 extracted from the compressor 81 or cooling medium 89 supplied from an appropriate supply source provided outside is supplied to a cavity 96 through an impingement cooling plate 92 in which impingement cooling holes 91 are formed. The cooling medium 89 comes into contact with the ring segment 90 so as to forcibly cool the ring segment 90. After that, it is fed through a cooling path 93 provided in the ring segment 90 so as to cool the ring segment 90 again and then discharged into the main stream gas 15 through a ring segment rear end 94.
The cooling path 93 may be formed in a circular, rectangular, wave-like or other shape in its cross section and the cooling path 93 is constructed by a plurality of holes extending in the axial direction disposed substantially parallel to one another along the periphery.
In the gas turbine, although the performance thereof is improved by increasing the temperature of combustion gas, and high temperature combustion gas directly strikes the vane portion, cooling air is supplied through the interior of the vane against a rise of the temperature of the combustion gas to suppress a rise in the temperature of the vane portion.
Thus, in a disc portion for supporting the aforementioned vane portion, a cavity is provided between the disc portion and gas passage for the disk so as not to directly contact the high temperature combustion gas, and then by supplying air having a higher pressure and lower temperature than in the gas passage to the same cavity, the combustion gas is sealed so as to prevent a rise in the temperature of the rotor.
In a conventional industrial gas turbine constructed according to such a philosophy, a structure for preventing a rise in temperature of the aforementioned disc portion by the high temperature combustion gas (high temperature gas, main stream gas) will be described with reference to FIGS. 10 and 11.
The high temperature gas A flows in the direction indicated by arrows 8 so that it passes the movable vane 1, static vane 3 and movable vane 2 in order from the upstream of the turbine to the downstream thereby forming a gas passage 7. At this time, if the high temperature gas A in the gas passage 7 (corresponding to the main stream gas 15 in FIGS. 1 and 16) invades in the upstream cavity 31 and downstream cavity 32 formed by the static vane 3, disc 4 and disc 5, and the temperatures of the disc 4 and disc 5 become higher than a tolerable value.
To prevent this phenomenon, sealing air B having a higher pressure and lower temperature than in the gas passage 7 is introduced from an outside diameter side of the static vane 3 into a static vane inside cavity 33 (hereinafter referred to as cavity 33) formed by the inside diameter side (or inside peripheral side) of the static vane 3 and a holding ring 38 disposed so as to oppose it and by supplying the aforementioned sealing air B from the cavity 33 in the direction indicated by an arrow 35 through a hole 34 open to the upstream cavity 31. Pressure of the upstream cavity 31 is kept higher than the pressure of the gas passage 7 so as to prevent an invasion of the high temperature gas A.
The sealing air B, which is supplied to the upstream cavity 31 in this way, passes through a clearance 6 formed by a sealing piece 9 mounted on the holding ring 38 and the discs 4, 5 sealed with a inter-disc seal 50. As shown in FIG. 10, the sealing air flows in a direction indicated by an arrow 36, so that the sealing air B is supplied from the upstream cavity 31 to the downstream cavity 32 which has a lower 9 pressure.
As a result, the pressure of the downstream cavity 32 is also kept higher than the pressure of the gas passage 7, corresponding to the downstream position relative to the static vane 3, so that an invasion of the high temperature gas A into the downstream cavity 32 is prevented.
However, if the clearance 6 is too large, the sealing air B more likely flows into the downstream cavity 32 so that a pressure of the upstream cavity 31 drops. To avoid this drop of pressure, it is necessary to keep the pressure of the upstream cavity 31 higher than the pressure of the gas passage 7. Thus, a larger amount of the sealing air B is necessary.
The sealing air B keeps the pressures of the upstream cavity 31 and downstream cavity 32 higher than the pressure of the gas passage 7 so as to prevent a rise in the temperature of the disc portion and then the air blows into the gas passage 7. Thus, after this blow, the sealing air B turns to waste air which carries out no work.
Therefore, although from the view point of efficiency, it is desirable to reduce the entire flow rate of the sealing air B as much as possible without expanding the aforementioned clearance 6, if the clearance 6 is too small, the sealing piece 9 and the discs 4, 5 come into contact with each other so as to produce damage therein, due to a difference in elongation by heat in non-steady state during gas turbine operation. Therefore, it is necessary to set and maintain a necessary but minimum clearance in which contact and damage are prevented and the flow rate of the sealing air B is minimized.
Next, as another conventional example, an airplane gas turbine will be described with reference to FIGS. 12-14.
The sealing air B is introduced from an outside diameter side of the static vane 3 into a static vane inside cavity 53 (hereinafter referred to as cavity 53) formed by a box 57 mounted on the inside diameter side of each of the static vanes 3, and is supplied through a hole 54 that is open to the upstream cavity 51 in a direction indicated by an arrow 55 so as to keep the pressure of the upstream cavity 51 higher than the pressure of the gas passage 7, thereby preventing an invasion of the high temperature gas A.
The box 57 is of a completely sealed structure except the hole 54 for feeding the sealing air B into the upstream cavity 51 so that the sealing air B does not leak from the cavity 53 directly to the downstream cavity 52.
Therefore, because all the sealing air B introduced into the cavity 53 is supplied into the upstream cavity 51, the leakage of the sealing air B is eliminated so that the amount of the sealing air B can be reduced.
Further, the amount of the sealing air B supplied from the upstream cavity 51 to the downstream cavity 52 in a direction indicated by an arrow 56 is limited by the clearance 6 formed by the sealing piece 9 mounted on an inside diameter side of the box 57 and the discs 4, 5.
The box 57 is directly mounted on the inside diameter side of the static vane 3, and because the static vane 3 is disposed in the gas passage 7 so that it is in contact with the high temperature gas A, the temperature change thereof is large and the change rate thereof due to thermal elongation is also large. Therefore, a displacement in the radius direction of the sealing piece 9 mounted on the same box 57 is governed by the box 57 and static vane 3 so as to become larger.
For this reason, the clearance 6 on assembly stage (initial stage) of the turbine needs to be set considering a thermal expansion amount 71 of the static vane 3 as shown in FIG. 14. And, therefore, the clearance 6 expands until the thermal expansion reaches its saturation (the amount of the sealing air B needs to be increased), so that the performance of the turbine at the time of partial load drops.
Further, because the thermal expansion amount of the static vane 3 is determined depending on the temperature distribution of the combustion gas that is likely to produce a deviation in temperature distribution, if a maximum of that deviation is considered, it is difficult to reduce the size of the clearance 6.
Although this thermal expansion produces only a slight influence in the case of an airplane gas turbine, because the diameter thereof is small, however if the same structure is applied to an industrial gas turbine, the displacement by the thermal expansion of the static vane in the industrial gas turbine, which has a large diameter is large as is indicated in FIG. 14 as a large displacement 72, and therefore becomes a problem that cannot be neglected.
As compared to the conventional industrial gas turbine, shown in FIGS. 10 and 11 with respect to this point, in the same industrial gas turbine, the holding ring 38 on which the sealing piece 9 is mounted is of a ring-like structure independent of the static vane 3 in the radius direction. Therefore, the displacement of the sealing piece 9 mounted thereon in the radius direction is governed by only the thermal expansion of the holding ring 38.
Further, because differences in diameter and temperature between the holding ring 38 and discs 4, 5 are small, the displacement of the clearance 6 in non-steady state is small as is indicated as a small displacement 73, and therefore, the clearance 6 at the time of assembly (initial stage) may be set small.
Further, because the displacement of the clearance 6 is independent of the thermal expansion of the static vane 3, which is largely influenced by a temperature distribution of the combustion gas, and therefore has a large displacement, it is not influenced by the expansion of the static vane. Therefore, the aforementioned displacement does not have to be considered and accordingly the clearance 6 may be narrowed. Thus, the flow rate of the sealing air B supplied from the cavity 31 to the cavity 32 can be reduced to a necessary but minimum level at all times including a partial load time.
In the cooling structure of the ring segment dispose d on the outside peripheral side in the radius direction of the movable vane of the above described conventional gas turbine, the cavity 96 needs to be kept under a higher pressure than the main stream gas 15 for the cooling medium 89 to prevent a backlash of the high temperature main stream gas 15. Therefore, the cavity 96, formed on the outside peripheral side of the ring segment, is kept under a relatively higher pressure than the main stream gas 15 on the upstream side in the axial direction, when the cooling medium 89 is supplied.
On the other hand, the pressure of the main stream gas 15 in the downstream side in the axial direction is lower than the pressure of the upstream side in the axial direction. Thus, the cooling medium 89 in the cavity 96, which has been adjusted of pressure in relation to the main stream gas 15 in the upstream side in the axial direction, produces excessive leakage so that a drop in the turbine efficiency is induced.
The temperature of the cooling medium 89 in the cooling path 93 is gradually raised by heat exchange for cooling as the medium flows downstream in the axial direction, so that it reaches a quite high temperature in the downstream of the ring segment, and therefore its cooling performance is reduced.
The structure of the sealing portion in the above-described industrial gas turbine has the following problems in reducing the flow rate of the aforementioned sealing air B.
Because the static vanes 3 are individually independent, a gap is produced between each vane. Even though a sealing plate 37 is provided, the gap is not completely closed by the sealing plate so that gaps 39, 40 are left, as shown in FIG. 10.
As a result, the sealing air B, introduced from the outside diameter side of the static vane 3 to the cavity 33 is supplied into the upstream cavity 31 through the hole 34 and at the same time leaks directly into the downstream cavity 32 through the gaps 39, 40 as leaking air C.
Thus, in the upstream cavity 31, the flow rate of the sealing air B supplied is insufficient so that the pressure thereof drops and the high temperature gas A invades the upstream cavity. To protect against this phenomenon, a larger amount of sealing air B is necessary in order to account for the flow rate of the leaking air C, so that the efficiency of the gas turbine drops.
On the other hand, in case of the conventional airplane gas turbine, although the generation of the leaking air C to the downstream cavity 32 as seen in the aforementioned industrial gas turbine has been avoided, there is a problem that the displacement of the clearance is governed largely by the thermal deformation of the static vane so as to become larger.
With respect to the relation between the cavity 33 of the industrial gas turbine or cavity 53 of the airplane gas turbine and each corresponding upstream cavity 31, 51 and downstream cavity 32, 52, each of the conventional industrial turbine and the airplane turbine have advantages and disadvantages, and therefore, it is difficult to determine which is better.